Tanqueray for gin&tonic, Sapphire for most other gin cocktails, and Hendrick's for the occasional better-than-sapphire martini.
If you're looking to try something different, track down a bottle of Kensington. It's aged in oak, and will be different from other gins you've tried. I'd recommend it in simple drinks - it makes a great martini, and you can drink it neat as well. You may have a little trouble tracking it down, but give it a shot if you see it. (It's tinged brown and looks like a a lightly-colored scotch, it's hard to miss.)
